US2023243A

US2023243A - Control system

Info

Publication number: US2023243A
Application number: US674894A
Authority: US
Inventors: Thomas R Rhea; Leonid A Umansky
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1933-06-08
Filing date: 1933-06-08
Publication date: 1935-12-03
Anticipated expiration: 1952-12-03
Also published as: GB426236A

Description

Dec. 3, 1935. RHEA r L 2,023,243

CONTROL SYSTEM Filed June 8, 1933 2 Sheets-Sheet 1 Figl.

- 3, 1935- T. R. RHEA ET AL CONTROL SYSTEM Filed June 8, 1933 2 Sheets-Sheet 2 VARIABLE SPEED DEVICE Fig. 2.

i 5 e W 6 K r a n mfimww fim fiA A mmw AQI w Tm T Patented Dec. 3, 1935 UNITED STATES PATENT OFFICE CONTROL SYSTEM Application June 8, 1933, Serial No. 674,894

12 Claims.

Our invention relates to control systems, more particularly to the speed control of machines of the type arranged for operation on a length of material, and has for an object the provision of such a control system which is reliable and eflicient in operation.

Although it obviously has other applications, our invention is particularly useful in connection with flying shears of the type used for severing bars of metal. These bars of metal are generally driven through the flying shear after the rolling mill has reduced them to predetermined dimensions.

Heretofore, the first crop from each piece of stock or length of metal has represented a loss because the length of the first crop has been indeterminate; that is to say, the stock may enter the shear while its' knives are in some indeterminate angular position. Therefore the length of the first crop may vary from a few inches to the full length of the cut. By the law of averages it may be assumed therefore that the average length of the first crop may equal approximately one-half the length of the cut. The loss in tonnage represented in the course of the days run becomes quite appreciable, and it is a further object of our invention to eliminate this loss.

It has been proposed to control the length of the first crop by stopping the rotary shear with the blades occupying a definite angular position after the cutting of each bar has been completed. The next bar as it approached the shear was arranged to control the energi'zation of the shear motor, so that the blades or knives of the shear occupied a predetermined position after the bar entered the shear. Much is yet to be desired in this type of control, because driving motors of very substantial sizes are required in order to accelerate the rotating portion of the shear to full speed in the time which-is available. The problem becomes increasingly more difilcult as the delivery speed of the stock is increased. The time for accelerating the motors is further diminished if the finishing stand is located close to the shears.v

In carrying out our invention, in one form thereof, a flying shear is driven continuously, and means are provided whereby the blades of the shear will occupy a definite angular position when the front end of a bar reaches a definite position in its travel. More specifically, we provide a position-determining device arranged to be accelerated from a definite angular position whenever the leading end of a bar of metal arrives at a predetermined distance from the shear. This definite angular position corresponds to the position that the blades should occupy. A second position-determining device is driven in angular agreement with the blades 5 of the shear. By means of a differential-positioning device the angular position of the blades is compared with the position which the blades should occupy. The diflferential-positioning device is arranged to accelerate or to retard temporarily the shear driving motor so as to eliminate the angular displacement, or in other words to bring the blades into the position which they should occupy. The differential-positioning device thereafter serves to maintain the speed of the shear so that the blades thereafter are maintained in the proper position to sever the bar into predetermined lengths.

It will be understood that in accordance with our invention the force involved to accelerate or go ity of the machines is obtained and the operatg ing costs are correspondingly less.

' For a more complete understanding of our invention reference should be had to the drawings wherein we have shown diagrammatically in Fig. 1 our invention applied to the control of a flying shear while in Fig. 2 we have shown a modified form of our invention.

Referring to the drawings, we have shown our invention in one form as applied to a direct current motor l0 connected by the gearing ii to 5 drive a flying shear l2. The shear l2 consists of two revolving drums l3 and it, each of which carries, respectively, the blades l6 and I1.

Steel bars or other types of stock, for example the bar in, are arranged to be driven from a rolling mill 22 to the rotary shear !2. The rolling mill 22 is driven by a motor 23. Inasmuch as the last operation in the production of steel bars of a given dimension is completed before the bar is cut into predetermined lengths, it will be understood that the, rolling mill 22 represents the last finishing stand. As long as the bar 20 is in the rolls of the finishing stand 22, the speed of the bar is determined entirely by the speed at which the motor 23 drives the rolls. However, after the go tail end of the bar 20 leaves the finishing stand 22 it is obvious that its speed no longer depends on the speed of the motor 23. Therefore, in order to maintain the accuracy of the cuts a pair of pinch rolls 25 are provided adjacent the shear l2. The pinch rolls 25 are drlven by the motor I 0 the connection including a variable speed drive 26 and the gears I l and 21. By shifting the belt 28 on the cone pulleys 29 and 38 of the variable speed drive the speed of the pinch rolls 25 may be adjusted to correspond to the proper delivery speed of the stock or bar 28. The length of each cut is determined by the ratio of the speed of the mill and of the shear l2, and after the bar leaves the finishing stand by the ratio of the speed of the pinch rolls 25 and of the shear l2. Therefore,.a, movable pointer 32 connected to the belt 28 is arranged to be movable over a calibrated scale 34, calibrated in terms of the length of out. It will be assumed that in the position shown the length of out is 10 feet. The speed of the motor 23 driving the rolls of the finishing stand 22 is now adjusted so that the delivery speed of the bar 20 from the finishing stand will correspond with the speed. at which the pinch rolls 25 will drive the bar through the shear. To assist in making this adjustment the speed indicator 40 is provided with a coil 4| connected in circuit with a tachometer generator 42. A second coil 44 on the indicator 40 is connected across the armature of a second tachometer generator 45 driven by the motor 23. The coils 4| and 44 act differentially on the pointer 46 of the indicator. When the speed of the rolls of the finishing stand corresponds to the speed of the pinch rolls 25, the pointer 46 occupies a. central position. As shown the speed of the motor 23 is adjusted by means of a rheostat 49 connected in circuit with the separately excited field winding 58 provided for the motor.

In accordance with the present invention the motor I0 is energized to drive the shear I2 continuously. Under these conditions in order to predetermine the length of the first crop made by the shear, it is necessary to insure that the blades l6 and I1 occupy a definite angular position (which angular position will insure a predetermined length of the first crop) as the leading end of the bar 20 arrives at the shear. It will further be understood that if the bar 28 is driven at a predeterminedspeed by the rolls of the finishing stand 22 and by the pinch rolls 25, the angular positions that the blades l6 and I! should occupy as the leading end of the bar 20 leaves the finishing stand can readily be determined. Therefore, the angular positions which the blades l6 and I1 sham occupy are compared with the angular positions that they actually occupy as the leading end of the bar leaves the finishing stand 22 and the difference, or the angular displacement, is eliminated by retarding or accelerating the motor l0 until the blades l6 and Il occupy the predetermined angular positions. Iri'this manner the length of the first crop is determined. In Fig. 1 this is accomplished by providing a position-determining device 55, the rotor of which is normally maintained in a predetermined position by means ofa disk 56 provided with magnetic inserts 51. The magnetic inserts are arranged to be attracted by the coils 58 so that the rotor of the position-determining device 55 is always brought to a standstill in a predetermined position. This means for bringing the rotor of the position determining device to a standstill in a predetermined angular position forms no part of our invention, and it is described and claimed in a copending application of Stewart H. White, Serial No. 674,890, filed June 8, 1933, assigned to the same assignee as this application.

The position-determining device itself is provided with a distributed three-circuit rotor .winding (not shown) and with a distributed threecircuit stator winding (not shown). Devices of this type are described and claimed in U. S. Patent 1,612,11'7-Hewlett et al.December '28, 1926.

The rotor of the device 55 is arranged to be connected to the motor 23 by means of a magnetic clutch 60 provided with an operating coil. 6|. Between the magnetic clutch and the motor 23 there is interposed a variable speed'mechanism 63 consisting of

cone pulleys

64 and 65, a belt 66 and a belt shifter 61. The variable speed mechanism 63 is driven from the motor 23 by means of the gears 68 The rotor circuit of the device 55 is connected to the stator circuit of a second device 18. This device has a single circuit rotor winding (not shown), the terminals of which are connected to a single phase source of supply represented by the reference numeral I2. This winding is mounted on a shaft 13 provided with an operating handle 14 whereby the inductive relation of the rotor winding with respect to the stator winding can be varied as may be desired. This device is described and claimed in U. S. Patent 1,637,039 to Hewlett et al. dated July 26, 1927. A position-sending device 16 is driven by the shear driving motor 10 and is of a construction similar to the device 18. Its single circuit rotor winding (not shown) is energized by the single phase, alternating current source of

supply

12,30 while its three-circuit stator winding is connected to the three-circuit rotor winding of a difierential receiving device 18. ,A three-circuit stator winding (not shown) of the device 18 is connected to the stator winding of the device 55. The position that the blades of the shear should occupy when the leading end of the bar 28 arrives at a predetermined distance from the shear I2 is determined by the device 55. When the bar reaches this predetermined point, the magnetic clutch 60 is energized to rotate the position-determining device 55 at the same speed as that at which the shears should rotate. This speed relation is obtained by means of the mechanism 63. In order to assist in fixing this speed a differential receiving device 80 is provided and has its threecircuit stator winding connected to the stator winding of the position-sending device 16, while its three-circuit rotor winding is connected to the three-circuit stator winding (not shown) of a device 82 provided with a single circuit rotor winding (not shown) arranged to be energized from the alternating current source of supply 12. The belt shifter 61 is operated until a pointer 84 driven by the rotor of the device 80 remains stationary. As long as there is a difference in speed between the rotors of the

devices

16 and 82 the pointer 84 will rotate. As soon as it remains stationary, however, an indication is had that the belt 66 has been properly adjusted. so A flag switch 86 is arranged to be operated to closed circuit position whenever the leading end of a bar, for example the bar 28, strikes the operating member 81 of the switch. A spring 88 normally biases the switch to the open circuit po- 65 sition. Assuming that the bar 20 has closed the switch 86, it will be observed that an energizing circuit is completed for the operating coil 89 of a relay 90. The relay 9!] is thereupon free to open its normally closed contacts 9| to interrupt a nor- 7 mally energized circuit including the coils 58 and to close its

contacts

92 and 93. Consequent to the closing of the contacts 92 an energizing circuit is completed for the operating coil 6| of the clutch 60. The clutch thereupon connects the rotor 7 of the position-determining device 55 to the variable speed mechanism. The rotor of the positiondetermining device 55 will be rotated at the same speed as the rotor of the position-sending device I6. It will be remembered that the rotor of the position-determining device 55 was held stationary at the angular position that the blades I6 and II should occupy when the leading end of the bar 20 operated the fiag switch. Consequently, if the blades i6 and I! are not in this predetermined angular position the rotor of the position-sending device 16 will be in a difierent position with respect to its stator winding than is the rotor of the device 55. Under these conditions it will be understood that the respective voltages applied to the difierential receiving device I8 are displaced with respect to each other, thereby moving the rotor of the device I8 an amount proportional to the angle of displacement between the two devices 55 and IS. The speed of the motor I0 is then varied until this angular difierence disappears. When the rotors of the devices 55 and I6 are in angular agreement with each other, the blades I6 and I1 occupy the predetermined angular positions which will result in a predetermined crop of the bar 20 when it arrives at the shear I2.

The energization of a separately excited field winding 98 provided on the motor I0 is controlled by means of a rheostat 99 and a motor generator set I00. A motor MI is energized from a suitable source of direct current supply. A generator I02 is provided with a pair of field windings I04 and I05. The field winding I04 is connected by the conductor I00 to one side of the secondary winding I08 of a transformer I09 provided with a primary winding H0. The other side of the field winding I04 is connected to the anode III of an electric valve H2. The field winding I05 is connected by conductor II3 to the other side of the transformer secondary winding I08. The other side of the field winding is connected to an anode III of an electric valve H8. The valves H2 and I I8 are preferably of the type having an inert gas, such as mercury vapor, within their envelopes. It will be understood that when the respective grids I20 and I2I of the valves H2 and H8 are made sufficiently positive with respect to the anodes, the valves become conductive.

The grid circuits of the valves include a translormer I23, the primary of which is energized from the single circuit winding (not shown) of the device I25, the stator winding (not shown) being connected to the contacts 93 of the relay and to the single phase source of supply I2. The rotor of the device I25 is mechanically connected to the rotor of the differential positiondetermining device I8. It will also be observed that the primary winding IIO of the transformer I09 is arranged to he energized by means of the contact 93 from the single phase source of supply I2. The cathodes I21 and I28 with their respective valves H2 and I I8 are energized from a suitable source of supply and are connected by means of the conductor I30 to the mid-point of the transformer secondary winding I08 and also by corductor i3I to the mid-point of the secondary winding of the transformer 23. The rotor of the device I25 norgnally occupies a position with respect to its stator winding such that no voltage is applied to the primary winding of the transformer I23. However, if an angular disagreement is indicated by the differential device I8 it -will, of course, be understood that the rotor of the device I25 is moved from this neutral position.

The consequent change in the relative positions of the stator and rotor windings causes a resultant voltage to be produced on the primary winding of the transformer I23. The polarity of this voltage is determined by the direction of rotation of the rotor winding. If, for example, the differential device 18 rotates the rotor of the device I25 in a clockwise direction, the voltage applied to the primary of the winding of the transformer I23 may cause a voltage to be produced on the grid I2I which will render the valve II8'conductive. The movement of the rotor of the device I25 in the opposite direction will then cause a voltageto be applied to the grid I20 of the valve I I2 to render this valve conductive. Accordingly, the field winding I04 will be energized in a direction to produce a. voltage of one polarity on the generator I02 and the field winding I05 is arranged to produce an opposite polarity on the generator. Assuming now that the rotor of the device I25 has moved in a clockwise direction to cause the valve H8 to become conductive, it will be observed that the field winding I05 will be energized. The generator I02 will thereupon produce a voltage in a direction to reduce the voltage which is normally applied to the field winding 98 of the motor I0 by the direct current source of supply. This causes the motor I0 to increase its speed. As soon as the angular disagreement between the position which the blades of the shear occupy and the position which they should occupy is eliminated, the differential device I8 moves the rotor of the device I25 to its neutral position, and the valve H8 again becomes non-conductive. The voltage of the generator I02 disappears and the shears again rotate at their normal speed. Assuming now that the angular disagreement is in the opposite direction so that the difierential device I8 operates the rotor of the device I25 in a counter-clockwise direction, it will be seen that the valve I I2 is rendered conductive. The resultant energization of the field winding I04 causes the generator I02 to produce a voltage which adds to that of the direct current supply. The increase in the excitation produced by the field winding 98 decreases the speed of the motor I0 until the angular disagreement has been wiped out.

In addition to temporarily varying the speed of the shear driving motor I0 the speed modifying means also serves to maintain a predetermined relation between the speed of the shear and the speed of the rolls of the rolling mill 22. For example, if the speed of either the shear driving motor I 0 or-the mill driving motor 23 should change after the bar has operated the flag switch 86, the differential device I8 will immediately indicate the change in the relative speeds. The electric valve speed controlling means will immediately function to vary temporarily the speed of the shear driving motor I0 so that the blades I 6 and H of the shears I2 always occupy their predetermined positions.

If the first crop taken by the shear happens to be too long or too short, the handle I4 of the device I0 is rotated an amount which is estimated' to insure the desired length of crop. The result of rotating the rotor of the device I0 with respect to its stator is to change the electrical relationship between the device 55 and the device I8 by causing an angular displacement of the voltage which is applied to the rotor of the device 55. This serves as the electrical equivalent of actually changing the angular position from which the rotor of the position-determining device 55 is accelerated. If'the crop of the first le gth of material is greater or less than that de red, it will be understood that the handle 14 is again rotated for an estimated correction. In ordinary practice it requires but a' few adjustments of the device 10 to obtain the desired length of the first crop.

Assuming now that the tail end of the bar 20 is leaving the flag switch it will be observed that the spring 88 operates to open the contacts 06a. The opening of these contacts deenergizes the operating winding 89 of the relay 90,'which relay after a. predetermined time interval operates to open its'contacts 92 and 93 and to close its contacts 9|. The opening of the contacts 92 deenergizes the clutch 60 while the opening of the contacts 93 deenergizes the speed controlling means.

Consequent to the closing of the contacts SI of the relay 60, the magnet coils 58 are energized,

which coils serve to stop the disk 56 with the magnetic member positioned between the coils. The apparatus is now in readiness for the next bar and functions in the same manner as we have just explained as soon as its leading end operates the flag switch 81.

The time delay openingof the relay 90 is accompiished by means of a short circuited coil I35. By delaying the opening of the

contacts

92 and 93 of this relay more time is obtained for correcting the position of the shears. This additional time might be necessary in case it is found desirable 'to locate the flag switch 81 not behind the finishing stand 22, as shown, but ahead of it.

It will be remembered that the pinch rolls were provided to maintain the speed of the bar 20 at proper value because the shears I2 were located a substantial distance from the finishing stand 22. If the pinch rolls 25 had not been provided the speed of the bar 20 would not have been fixed during its whole travel from the finishing stand 22 to the shears I2. I

In Fig. 2 we have shown our invention in one form as applied to a rolling mill wherein the shears I2 and the finishing stand 22 are spaced apart a short distance. Accordingly, the pinch rolls 25 have been omitted and the motor I0 used to drive the rolls of the finishing stand 22, is also used to drive the shear. As shown, the preceding stand I52 is located a substantial distance from the last finishing stand 22 so that suflicient time will elapse during the travel of the bar from one stand to the other to permit the adjustment of the shear blades. Assuming that the speed of the motor 23 driving the stand I52 is set at a predetermined speed with reference to rolls 22 and therefore to the shears I2, it will be observed that as, the leading end of the bar 20 enters the stand I52 a relay I54, responsive to the current of the motor 23, is operated to interrupt by means of its normally closed contacts I55 an energizing circuit for a coil I56 provided on a timing relay I51 and to complete a circuit through its contacts I58 and through contacts I59 of the timing relay a circuit for the operating coil of the relay 90. The timing relay I51 is provided with a short ciroulted coil I60 which serves to maintain the timing relay closed for a time which approximates the time required for the bar 20 to travel from the stand I52 to the-stand 22.

The position-determining device 55 is arranged to be operated from a predetermined position as in the case of Fig 2. In this form of our invention, however, a direct current generator I62 provided with a separately excited field winding I63 adjusted by means of its field rheostat I66. In I i this manner the speed at which the position-determining device 55 rotates can be adjusted so as to correspond to the speed of the shears I2.

The speed control means includes an electric valve I10 having its output circuit connected 10 through a variable resistance I1I, contacts 63 of the relay 90, field winding I12 of the generator I02, secondary winding I13 of transformer I14 and to the cathode I15 of the valve I10. Preferably the field winding I12 is connected so that iii the generator I02 serves to increase the field excitation of the motor I0. The direct current output of the valve I10 is controlled by means of the device I25 provided with a single circuit rotor winding and a three circuit stator winding. By 2 means of a phase converting apparatus, shown ascapacitor I00 and reactor I82, the three circuit winding is directly connected to the single phase source of supply 12.

This manner of controlling the electric valve 25 is fully described in U. S. Patent 1,655,036 to Alexanderson et al., January 3, 1928 and in this specification will be briefly described by saying that the time of starting of current in electric valve I10 may be controlled for each half cycle. Therefore; the average flow of current which may flow through the field winding I 12 is directly proportional to the diflerence in the angular positions of the rotors of the

devices

55 and 16. It follows that the change in speed of the driving motor I0 caused by the voltage generated by the booster generator I02 also varies in proportion to the angular disagreement between themtors of the devices 55 and 16.- For example, if there is a substantial disagreement between the 4 device 55 and the device 16 after the clutch 60 has operated to accelerate the device 55 from the predetermined position, it will be understood that the voltages applied to the differential receiving device 10 are displaced a corresponding amount. Consequently, the differential device 18 rotates the rotor winding of the device I'25 an amount proportional to the displaced voltages. The result is a shift of the phase of thegrid voltage of the valve I10 with respect to its anode voltage so that the valve I10 becomes conductive over a substantial part of each half cycle. The booster generator I02 thereupon increases the excitation of the field winding 98 of the motor I0,

thereby decreasing its speed a substantial amount. As the blades I6 and I1 of the shears I 2 are operated into angular position the increased excitation produced by the generator I02 diminishes. When the blades reach the positions which they should occupy as determined by the device 55 and the setting of the device 10, the motor I0 is again rotating at its former speed but the shear blades I6 and I1 are now in their corrected positions.

While we have shown a particular embodi- 65 ment of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made, and we, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. Means for controlling a machine arranged to perform operations on a moving strip of material i ment between the shear and said device for temso that its operations occur at predetermined points on said strip, comprising driving means for said machine, driving means for said strip, means responsive to the position of said strip for indicating the desired angular position of said ma chine when said strip is a predetermined distance from said machine, and means controlled by said last mentioned means for temporarily varying the speed of said machine with respect to said strip so as to establish said desired angular position of said machine.

2. The combination with a'rotary shear arranged to sever strips of material into predetermined lengths, of means for continuously driving said shear, material driving means located a predetermined distance from said shear for driving said material to said shear, means for indicating a desired angular position of said shear when the leading end of said material is a predetermined distance'from said shear, and means controlled by said last mentioned means for varying the angular position of. said shear with respect to the leading end of said material until said desired angular relation is obtained.

3. The combination with a rotary shear arranged to sever strips of material into predetermined lengths, of means for continuously driving said shear, means located a predetermined distance from said shear for driving said strip of 7 material to said shear, a position-determining device normally at rest in a definite angular position, means responsive to the travel of said strip for accelerating said device to the speed of said shear, means responsive to the angular. disagreeporarily modifying the speed of said shear until said device and said shear are brought into angular' agreement and for maintaining said angular agreement.

4. Means for predetermining the length of the first cut of a strip of material arranged to be driven through a pair of rotary shears comprising a motor for driving said shears continuously, a position-determining device, a pilot motor arranged to accelerate said device from a predetermined angular position to the speed of'said shears, and means responsive to the angular difference between said device and said shears for controlling the speed of said shears so as to bring said device and said shears into angular agree,- ment.

5. Means for controlling a machine arranged to perform operations on a strip of material so that its first operation occurs at a predetermined point on said material comprising motor driving means for said machine, material driving means for said strip, a position-determining device, means for producing an electromotive force upon positional disagreement of said machine and said device corresponding in value and direction to the amount and direction of. said positional disagreement, and a pair of electric valves responsive to said electromotive force for controlling said motor driving means so as to bring said machine into positional agreement with said position-determining device.

6. Means for predetennining the length of the first crop of a strip of material arranged to be driven through a rotaryshear provided with a pair of cutting blades, comprising a motor for driving said shear, speed control means for said motor, a pair of driving rolls arranged a predetermined distance from said shear to drive said strip to said shear, means for driving said rolls, a position-determining device normally at rest in an angular position corresponding to the angular position which the blades of said shear should occupy when said strip enters said rolls, a pilot motor arranged to accelerate said position-determining device to a speed corresponding to the speed of said shear, means responsive to the arrival of said strip between said driving rolls for connecting said pilot motor and said positiondetermining device, a position sending device driven by said shear, a differential position-receiving device responsive to said position-determining device and to said position setting device for operating said speed control means so as to bring said shears into positional agreement with said position-determining device.

7. Means for controlling a machine arranged to perform operations on a strip of material so that its operation occurs at a predetermined point on said material comprising motor driving means for said machine, driving means located a predetermined distance from said machine for said strip, a position-determining device, means for accelerating said position-determining device from a predetermined angular position to the speed of said machine-whenever said strip of material enters said driving means, timing means therefor arranged to decelerate said positiondetermining device after a predetermined interval of time, and means for stopping said positiondetermining device in a predetermined angular position.

8. In a system for maintaining a predetermined relationship between a length of material and an element operating thereon, the combination of driving meansv for said material, a position-determining device, a driving motor therefor, a generator mechanically connected to said driving means, connections for connecting said generator in a local circuit with said driving motor so that the speed of said motor will vary directly with variations in the speed of said generator, a driving means for said operating element, and means actuated in accordance with the position of said element and cooperating with said position determining device for controlling one of said driving means to establish a predetermined positional relationship between said length of material and said element.

9. The method of predetermining the length of the first crop of a strip of material arranged to be driven through a rotary shear which consists in determining the angular position of said shear with respect to the leading end of said material which will establish the desired length of crop, comparing the actual position of said shear therewith when said strip is a predetermined distance therefrom, and varying the angular position of said shear with respect to said leading end of said strip until the desired angular position is obtained.

10. The method of establishing a predetermined relationship between an apparatus arranged to perform operations on a moving strip the combination of a position-sending device driven by each of said objects, a differential position-receiving device electrically connected to said sending devices, means for stopping one of said position-sending devices in a predetermined angular position, and means for varying the electrical relationship between said one sending device and said receiving device.

THOMAS R. RHEA. LEONID a. U'MANSKY.